1. Field of the Invention
The invention relates to a method of taking measurements along an optical transmission path by means of an optical sensor. The optical sensor includes an optical converter whose optical attenuation properties are influenced according to the value to be measured. A first beam portion of a high-frequency amplitude-modulated optical transmission beam is passed through the optical sensor. A second beam portion of this transmission beam is conveyed via a delay element. The second beam portion is transmitted together with the first beam portion, which is outputted by the optical converter to the receiver arrangement via the transmission path as a receive beam. In this way a modulation change produced by the sensor is processed and evaluated as information about the measured quantity.
2. Prior Art
Such a method is disclosed in "Proceedings of the 2nd International Conference on Fiber Optic Sensors", Stuttgart 1984, pages 378 to 390. In this prior art, one of the beam portions of the transmission beam, which is not affected by the converter of the sensor, is passed through an optical delay line. After a time shift this beam portion is combined again with the converter-influenced beam portion. Together the two beam portions are fed into an optical wave guide used for transmission to a receiver arrangement. The beam portion conveyed through the delay line constitutes a reference beam. The relative phase angles of the modulation portion of the two beam portions are different. The resultant phase angle of the received beam is compared with the original phase angle of the transmission beam. Information about the value to be measured can be derived from the differential angle or the phase shift. Such information is independent of any fluctuations in the attenuations of the transmission paths from and to the transmitter. It is prerequisite that the overall length of the transmission path from the light source to the sensor and from the sensor to the receive arrangement is known, as only when this condition is satisfied can the measuring value can be determined from the differential angle. When the overall length of the optical transmission paths is altered the receive arrangement must be recalibrated.
The invention has for its object to provide a method of the type described in the opening paragraph in which it is possible to measure a value at the transmitter end for any optical transmission paths of undefined lengths without recalibration of the receive arrangement.
This object is accomplished by having the optical transmission beam be additionally subjected to low-frequency modulation. An original modulation factor m.sub.0 of the transmission beam is equal to the ratio of the low-frequency (I.sub.NF) to the high-frequency (I.sub.HF) modulation amplitude. The original modulation factor m.sub.0 is fed as data to the receive arrangement 3. The sensor-influenced received modulation factor, ##EQU1## of the receive beam is measured by the receive arrangement 3. The received-to-original modulation factor ratio is evaluated as information about the value to be measured.
Theoretical considerations show that the ratio between the modulation factors of the two beam portions depends not only on the attenuation of the transmission path, but also on its length. By means of the dual modulation effected in accordance with the invention, with significantly different frequencies, it is possible to reliably determine from the received optical signals the modulation factor m.sub.1 changed by the transmitter, without the risk of the results being degraded by the dark current of a photodetector.
To ensure that for the low-frequency modulation there is substantially no phase difference between the two beam portions, it is advantageous for the modulation frequencies of the high-frequency and low-frequency modulations to differ in ratio by at least 10.sup.3.
In accordance with an advantageous embodiment of the invention the high-frequency modulation signal is multiplied by the low-frequency modulation signal and the received modulation signals are separated by filters and their peak values are determined.
If the modulation amplitude of the high-frequency modulation is chosen to be twice the modulation amplitude of the low-frequency modulation, it is possible to obtain with simple circuit means an original modulation factor m.sub.0 which is always constant.
Particularly simple circuits can create square-wave modulation.
Simple circuit structures may also be used when the high-frequency modulation is only superimposed on the positive range of the low-frequency modulation. It is then not necessary for the period of time of the positive half-wave of the low-frequency square-wave modulation to be equal to the period of time of the negative half-wave. It is, however, advantageous for the superimposed high-frequency modulation to be formed from symmetrical positive and negative half-waves. This is basically ensured for square-wave modulations, independent of instantaneous curves in the characteristics of a LED.
It is particularly simple to form a known and uniform modulation factor when the transmitter transmits groups of optical square-wave pulses having a duty cycle of 1:1, and the transmission time of a group is followed by an interval whose length amounts to a multiple of the period of the square-wave pulse.
Using the method according to the invention a particularly high measuring sensitivity is achieved when the length of the delay line is approximately an odd multiple of half a wavelength of the high-frequency modulation.